This invention relates to the production of coal in situ, wherein a series of wells is drilled into an underground coal seam. More particularly the invention relates to determining the paths of maximum permeability within the coal by recovering oriented cores, partially dewatering the coal by selectively removing the water by removal to the surface of the earth, then establishing reaction zones in the coal.
Production of coal in situ is a well known art, having been practiced experimentally in the United States and many other countries of the world, and having been practiced commercially in Russia. With a rather considerable amount of data generated from the various in situ coal projects, common problems that require solutions are relatively easy to define.
Since it is imperative that fluids be made to flow through the coal seam, effective permeability of coal is an important consideration. All coals have a measure of permeability with the maximum permeability being along the cracks and fissures within the coal seam. Coal, being a non-homogeneous rock, may or may not have predictable patterns of permeability. Existing patterns of permeability may be determined by the cut-and-try methods of drilling a series of wells into the coal seam, then injecting fluids under pressure selectively between pairs of wells to determine which wells have natural linkages. An improvement over these methods is described in the present invention.
Coal seams located above the natural water table underground and generally classified as dry seams whose water content is substantially limited to the chemically combined water within the coal itself. Coal seams located below the natural water table underground generally are classified as wet seams, although in some cases the coal itself may be relatively dry of free water. Such seams can be converted into true wet seams by the simple expedient of innerconnecting the seam to an overlying aquifer. Many coal seams are natural aquifers, with the water charge source being at an outcrop of the seam. Coal seams that are aquifers are good candidates for production using in situ techniques because by being an aquifer the particular seam has demonstrated permeability, otherwise water could not percolate through the seam.
It is desirable that a coal seam selected for in situ production be a wet seam. Free water in the seam provides a hydraulic seal against the escape of gases generated by in situ techniques. Free water in the seam also provides a safety measure to preclude the possibility of runaway burns underground. While free water is an advantage for in situ techniques, control of free water into the underground reaction zone is required. The reaction zone is established in the underground coal by setting the coal afire and propagating the fire by controlled injections of a suitable oxidizer, then withdrawing the products of the reactions to surface facilities. Initial ignition of the coal is difficult if considerable free water is present. Then after ignition, if considerable free water is present, efficiencies of in situ techniques are seriously impaired by the amount of heat required to convert the water to vapor so that it may be removed along with other products generated by in situ processes.
One approach to the water problem underground is described in U.S. Pat. No. 2,973,811 of Rogers, wherein methods are taught to locate the water in an underground carbonaceous deposit by using electrical resistivity surveys. Once the water is located and mapped, plans are made to drive the water out of that portion of the carbonaceous stratum where it is desired to engage in combustion in situ. The water then is displaced by injecting a gas such as air, such displacement forcing the water radially outward from the injection well bore. Such displacement, and the maintenance of the water in its displaced location, requires considerable increase in the normal formation pressure. Such procedures also require a considerable amount of competent overburden to avoid ruptures to the surface of the ground and the consequent loss of hermetic seal required for in situ techniques.
Another approach to the water problem underground is taught in United Kingdom Patent No. 697,189 of Williams. Methods are taught for use in a steeply dipping coal bed wherein two shafts are sunk to the coal seam and the shafts are interconnected with a horizontal channel serving as a reaction zone in the coal. Water is pumped continuously through the shafts to the surface of the earth during ignition and during combustion until a substantial amount of coal is consumed. To minimize the oxygen by-pass problem of greatly enlarged underground channels, the pumping action is slackened off to allow the water table to rise through the accumulated ash and rubble to plug a portion of the underground channel to the flow of gases. Such an arrangement requires a considerable amount of underground workings and requires pumping of water from the reaction zone itself.
Improvements over the methods of the prior art can be made in the case of determining the paths of permeability in the coal by taking oriented cores of the coal for inspection above ground, and in the case of water control by drawing down the water table in the immediate vicinity of the reaction zone rather than limiting the drawdown to the reaction zone itself. It is an object of the present invention to teach such improvements. Other objects, capabilities and advantages of the present invention will become apparent as the description proceeds.